1. Field of the Invention
The present invention relates to a hydraulic circuit of a hydraulic construction machine such as a hydraulic excavator.
2. Description of the Related Art
For example, in the hydraulic excavator, a structure has been used that a hydraulic pilot type control valve is actuated by an operation of an operation means (hereinafter, as an example, a remote control valve is described) to actuate a hydraulic actuator.
In such a structure, a bleed-off control is performed that based on an operation amount of the operation means, a flow rate of pouring from a pump to a tank (that is, a supply flow rate to a hydraulic actuator) is changed.
With respect to the bleed-off control, for example, as disclosed in Japanese Unexamined Patent Application Publication No. 11-303809, a technique of using a common bleed-off control system in which a bleed-off valve for bleed-off is commonly used for a plurality of control valves (hydraulic actuators) has been known.
In the common bleed-off control system, an operation pilot pressure of a remote control valve is detected by a pressure sensor and is sent to a controller. From the controller, a command current is given to an electromagnetic proportional pressure reducing valve (hereinafter, referred to as a proportional valve according to its common name) so as to change the secondary pressure, and with the secondary pressure, an opening area of the common bleed-off valve, which is a spool valve of hydraulic pilot type, is changed.
However, between the operation amount and the proportional valve, difference is generated in each machine due to the instruction current and a tolerance of the secondary pressure of the proportional valve. That is, in each opening area of the bleed-off valve with respect to the same operation amount, individual difference in each machine is generated.
In such a case, behavior of the hydraulic actuators (for example, starting points) with respect to an operation differs in each machine, and the operationality becomes worse.
Accordingly, it is desirable to correct the relation between the operation amount and the proportional valve secondary pressure in each machine.
As a technique related to the above, Japanese Unexamined Patent Application Publication No. 2-134404 has been known. In this publication, in a hydraulic circuit that a proportional valve is actuated in response to a command current from a controller based on an operation amount and a control valve is controlled by its secondary pressure, the secondary pressure of the proportional valve is detected by a pressure sensor and the detected pressure is fed back to the controller. Then, the command current value is corrected to compensate for the displacement of the change of the secondary pressure with respect to the change of the command current.
Then, it is possible to apply the correction technique to the technique for correcting the characteristics of the command current value (operation amount) and the proportional valve secondary pressure.
However, if the technique disclosed in Japanese Unexamined Patent Application Publication No. 2-134404 is directly used, to the existing circuit, new equipment, that is, the pressure sensor for detecting the proportional valve secondary pressure and the wiring equipment between the sensor and the controller, should be added. Then, the cost increases and it is difficult to additionally install the equipment to the existing device.